Cardiac Flashcards
What is the threshold of the SA node
-40mV
What is the resting potential of a ventricular fiber
-80 to -90
What is the resting membrane potential of the SA node
-55 to -60mV
The SA node is composed of what
Special cardiac muscle fibers
The SA node connects to what
Directly to the Atrial fibers
The AV node receives signal form the SA node ____ after origin
.03 seconds
How long is the signal delayed in the AV node
.09 seconds
Why is signal delayed in the AV node
due to small size of cells, low amplitude of AP, and slow rate of depolarization during excitation
How long is the final delay of signal in the penetrating bundles of the AV node
.04 seconds
How long is the overall delay from the initial origin of the signal until onset of ventricular contraction
.16 seconds
The slow conduction in the conductive system of the heart is caused mainly by ______ resulting in an increase in the ________
Diminished number of gap junctions along the pathway, resulting in an increase in the resistance to conduction
What is the resting membrane potential of the SA node fiber
-55 to -60 mV (Threshold= -40mV)
Why are fast sodium channels in the SA node already inactivated at resting potential
Inactivation gates close when membrane potential is less negative than -55mV
- The SA node fiber has a resting membrane potential of -55–60mV
Why is atrial nodal AP slower to develop than ventricular
Only slow sodium-calcium channels can open and the fast sodium channels are inactivated due to the resting potential of -55- (-60)
When do the sodium-calcium channels become activated
at -40mV
how long are the sodium-calcium channels in the SA node open
they become inactivated within 100-150msec after opening
When do the potassium channels in the SA node become open
at the same time as when the sodium-calcium channels become inactivated
How long do potassium channels remain open in the SA node
for a few tenths of a second
What is the ventricular fiber resting potential
-85 to -90
The atrial nodes do note have what two phases
phase 1 & phase 2
The AP originating in the SA node generate what
The “sinus rhythm”
What are action potentials that originate anywhere else than the SA node
Ectopic focus or pacemaker
Where is the Vagus nerve main distribution in the heart
The SA and AV nodes
What is the neurotransmitter used by the Vagus nerve
Acetylcholine
What is a muscarinic receptor
A receptor that is located on the SA and AV node that is stimulated by the Vagus nerve
What effect does the Vagus nerve have on the SA node
it decreases rate of rhythm (negative chronotropic effect)
What effect does the Vagus nerve have on the AV junctional fibers
slowing transmission of the cardiac impulse into the ventricles
What effect does the vagus nerve have to cardiac fiber membrane permeability
it increases permeability of the fiber membranes to potassium ions
- hyperpolarization: -65 to -75 rather than -55 to -60 mV
Where is the sympathetic innervation of the heart distributed
to all parts of the heart, mainly the ventricles
What is the neurotransmitter used by the sympathetic nervous system
Norepinephrine
What effect does norepinephrine have on the heart
stimulates beta-1 adrenergic receptors
- increases depolarization rate (positive chronotropic effect)
- may increase permeability of fiber membranes to sodium and calcium ions
What does the P wave on an EKG represent
Atrial depolarization
What does the QRS on an EKG represent
ventricular depolarization
What does the T wave on an EKG represent
Ventricular repolarization
What do ECG measure
The extracellular potential: it is not the same as the potential recorded form an axon when recording the transmembrane potential
What is an ECG
A graphic representation of the electrical activity in cardiac muscle tissue produced by regions of depolarization and repolarization
When does deflection from 0 on an ECG occur
only when there is current flow between regions of the heart (when there is variation in the membrane potential in different regions of the heart.) Current flows between regions of different membrane potentials.
When does current on an ECG not occur
when only the atria and ventricles have different potentials
Ventricular muscle has a _______ action potential
Monophasic
The QRS marks the beginning of what
ventricular AP
The T wave marks what
The end of the ventricular AP
No AP is recorded when the ventricle is ______
completely polarized or completely depolarized
Only when the muscle is partly polarized or partly depolarized does ________
Current flow from one part of the ventricle to another
Repolarization of the ventricles occurs
after the end of the T wave
what is the P-Q (P-R) interval
beginning of P wave—-> beginning of QRS
- is about 0.16 seconds
note that the 0.16 seconds is the same amount of time that an AP gets from the SA node to the ventricular fibers due to the delay system in the AV node.
how long is the Q-T interval
0.35 seconds
What is the Q-T interval
it is the time from ventricular depolarization to ventricular repolarization
The P wave represents what phase of the cardiac cycle in the atria
Phase 0 (rapid depolarization)
The T-wave represents what phase of the cardiac cycle in ventricles
phase 3 (repolarization)
The QRS represents what phase of the cardiac cycle in ventricles
Phase 0 (rapid depolarization)
The T wave represents what phase of the cardiac cycle in ventricles
Phase 3 (repolarization)
In a typical ECG, which of the following waves occurs at the beginning of the contraction of the atria
P wave
what is typically not seen on an ECG
Atrial repolarization
Which of the following is the direction of bipolar lead III
120 degrees
Of the three bipolar leads, which one is connected to both left and right arms
Lead I
What is the mean electrical axis of the normal ventricles
it is 59 degrees
what is the amplitude of the SA node
about 60mv
resting potential is -55 to -60 and the peak is about
